1. Field
Example embodiments relate to an organic semiconductor polymer having liquid crystal properties, an organic active layer, an organic thin film transistor (OTFT), and an electronic device including the same, and methods of preparing the organic semiconductor polymer, and fabricating an organic active layer, an OTFT and an electronic device using the same. Other example embodiments relate to an organic semiconductor polymer having liquid crystal properties, in which fused thiophenes having liquid crystal properties and aromatic compounds having N-type semiconductor properties are alternately included in the main chain of the polymer, thus simultaneously imparting both increased charge mobility and decreased off-state leakage current, an organic active layer, an organic thin film transistor (OTFT), and an electronic device including the same, and methods of preparing the organic semiconductor polymer, and fabricating an organic active layer, an OTFT and an electronic device using the same.
2. Description of the Related Art
In general, an OTFT, including a substrate, a gate electrode, an insulating layer, source/drain electrodes, and a channel layer, is classified into a bottom contact (BC) type, in which the channel layer is formed on the source/drain electrodes, and a top contact (TC) type, in which a metal electrode is formed on the channel layer through mask deposition.
The channel layer of the OTFT may be formed of an inorganic semiconductor material, for example, silicon (Si). However, with the fabrication of relatively large, inexpensive, and flexible displays, the use of an organic semiconductor material, instead of an expensive inorganic material requiring a high-temperature vacuum process, may be required.
Research has been directed to organic semiconductor material for the channel layer of the OTFT, and the transistor properties thereof have been reported. Examples of low-molecular-weight or oligomeric organic semiconductor materials may include merocyanine, phthalocyanine, perylene, pentacene, C60, and thiophene oligomer. According to the related art, the use of pentacene monocrystals may result in increased charge mobilities of 3.2˜5.0 cm2/Vs and above. In addition, relatively increased charge mobility of 0.01˜0.1 cm2/Vs and on/off current ratio using an oligothiophene derivative have been reported. However, these techniques may be dependent on a vacuum process for the formation of a thin film.
Further, there may be OTFTs using an organic semiconductor polymer material, which is exemplified by a thiophene polymer. Although these OTFTs may have properties inferior to OTFTs using low-molecular-weight organic semiconductor material, processability may be improved because a relatively large area may be realized at decreased expense through a solution process, for example, a printing process. In this regard, the related art has reported the experimental fabrication of a polymer-based OTFT using a polythiophene material, called F8T2, leading to charge mobility of 0.01˜0.02 cmM2/Vs. As mentioned above, the organic semiconductor polymer material has TFT properties, including charge mobility, inferior to those of low-molecular-weight organic semiconductor material, including pentacene, but the organic semiconductor polymer material may eliminate the need for an increased operating frequency and may enable the inexpensive fabrication of TFTs.
With the goal of commercializing OTFTs, an increased on/off current ratio may be an important characteristic to be realized in addition to charge mobility. To this end, off-state leakage current may be minimized or reduced. The related art discloses an OTFT including an active layer composed of n-type inorganic semiconductor material and p-type organic semiconductor material to thus slightly improve the characteristics of the OTFT, which is nevertheless difficult to use in mass production because the fabrication process is similar to a conventional Si-based TFT process requiring deposition. In addition, the related art discloses an OTFT having charge mobility of 0.01˜0.04 cm2/Vs using regioregular poly(3-hexylthiophene) (P3HT). In the case of using poly(3-hexylthiophene) (P3HT) as a typical regioregular material, the charge mobility may be about 0.01 cm2/Vs, but the on/off current ratio may be relatively low, e.g., as low as about 400 or lower, due to the increased off-state leakage current (about 10−9 A and above), and consequently the above material is limited in application to electronic devices.
Moreover, organic semiconductor materials having liquid crystal properties capable of realizing increased charge mobility have been studied. For example, the related art discloses a semiconductor material containing (2,3-b)-thienothiophene to thus improve charge mobility, processability, and oxidation stability. Although such an organic semiconductor polymer material may function to increase charge mobility, the pi-pi conjugation length of the organic semiconductor polymer may not be effectively controlled, undesirably making it difficult to satisfy both increased charge mobility and decreased off-state leakage current.